When you visit a site with a Facebook 'Like' button, a whole lot is happening in the background.
Whether you click it or not - and 'Like' things or not, the presence of the button allows Facebook to link your off-Facebook browsing activity to your profile.
Fundamentally, this is taking advantage of a technical 'trick' to collect data about users without their explicit knowledge or permission. Google's '+1' button does pretty much the same thing.
The bargain is fairly one-sided though, because so much of Facebook's content is behind the 'login wall'. Search engines can't get to it, meaning Facebook get all the off-site data for their social graph - without giving much of their content back to the open web.
Facebook is breaking the web.